The present invention relates to a device for sheet conveyance, into a converting press for paper or cardboard sheets comprising at least one feeding station, a converting station, a waste stripping station and a delivery station for converted sheets, said conveyance device including two endless gripper bar chains assembled for conveying the sheets from the feeding station to the delivery station, a transverse driving shaft equipped with driving wheels for the endless gripper bar chains and at least one device for sheet capture secured to the endless gripper bar chains.
The wording xe2x80x9ctransversexe2x80x9d here means a horizontal direction, perpendicular to the machine axis.
Concerning known converting presses, only one electric motor usually drives the whole machine. This motor directly actuates inertia flywheels, a clutch brake device being inserted between the flywheels and the other machine bodies. This system drives all elements functioning with synchronism, in particular the movable beam of the platen press, the waste stripping and blank delivering stations, as well as the chains bearing the gripper bars ensuring sheet capture and conveyance from one station to the next.
A sheet conveying and converting cycle includes a sheet stop phase during which a given sheet is conveyed to a converting operation, such as blanking or waste stripping, and at least one moving phase during which the sheet is conveyed from one station to the next. This moving phase necessarily includes an acceleration and a deceleration phase and, usually, between both, a phase during which the sheet moves at a constant speed.
Various embodiments carrying out this kind of cycle have been described, in which the wheels driving the chains are interdependent in rotation with a coupling unit. The coupling unit is alternately caught or released from a driving unit by axial displacement. Standby means release or immobilize the wheels driving the chains, although the driving unit is alternatively driven in one or another rotative direction.
Such mechanical devices were described for example with patents CH 219422 and CH 411555. For such devices, an oscillating toothed segment operates on the transverse driving shaft via a pinion. The toothed segment is actuated by a rod connected to an eccentric secured on the top of a shaft driven by the general machine driving device. A complete rotation of the machine driving device causes an entire back and forth run of the oscillating toothed segment.
This kind of mechanical drive deals with a single motion law, determined by the geometry of the parts. This kind of driving device is very suitable for low or average conveyor rates up to approximately 5,000-6,000 sheets/hour. Beyond these rates, accelerations and decelerations at the beginning and at the end of the motion phase become very strong. However, after the sheet blanking operation, the sheets are connected only by their nicks, which can break in the case of heavy acceleration, causing a machine jam.
Several mechanical devices have been proposed to overcome this defect. The patent CH 411555 suggests driving the toothed segment under the control of a double cam. One of the double cams sends a rocking motion to a lever which, by means of a connecting rod, drives the toothed segment. Another of the two lever elements cooperating with the two cams sends the back free motion to the toothed segment, and is elastically engaged against the cam. The cam system modifies the effect of the motion law by relieving the start-up forces. However, for a set of reference cams, the acceleration forces are relieved for a set range. If other ranges of operation concerning the motion are desired, the cams must be changed.
Another known device attempts to remove the time delay for the blanking operation. In one such device, the manufacturer provided two blanking station platens that travel together in linear motion with the blanked sheets. This solution removes the time delay involved in the blanking operation, while permitting modulation of the linear displacement motion of the endless gripper bar chains.
The aim of the present invention is to provide a device for sheet conveyance allowing high throughput rates while carrying out an optimal sheet conveying cycle without any overly strong accelerations that could potentially break the nicks between the sheets blanks. Another aim of the invention is to allow rate changes for the sheet conveying cycle, independently from the conveying cycle of the converting station. The rate modification of a sheet conveying cycle involves the conveyor acceleration and deceleration curves and the respective duration of a cycle phase according to the type of processing carried out. The rate of the sheet conveying cycle can be changed between different processing jobs without having to retool or change parts.
These aims are achieved by a sheet conveyance device according to the present invention. A sheet conveyance device for a converting press for paper or cardboard sheets is provided. The press comprises at least one feeding station, a converting station, a waste stripping station and a delivery station for converted sheets. The conveyance device includes two endless gripper bar chains arranged for conveying the sheets from the feeding station to the delivery station, a transverse driving shaft equipped with driving wheels for the endless gripper bar chains and at least one device for sheet capture secured to the endless gripper bar chains. The transversal driving shaft of the conveyance device is driven separately from the other press stations, by at least one independent motor. The independent motor can operate with driving cycles comprising at least one motion phase and one phase of deceleration and/or stops controlled by a control device. The duration of each independent motor driving cycle is equivalent to the duration of a converting cycle of said converting station.
The driving of the sheet conveyance device by a motor independent of the motor driving the other converting press stations was initially thought to be an inadequate model, because of the problem of proper synchronization between the sequential sheets and the blanking platen operation of the downstream stations. The inventors noted to the contrary that an appropriate command, particularly an electronic command, can properly synchronize the conveyor with a platen cycle. The drive command is synthesized from a representative signal for the blanking station platen location and a representative signal for the gripper bar chains location. The appropriately synthesized drive command can control the independent motor driving the gripper bar chains to run with optimal synchronism related to the platen cycle. During a conveying cycle, a control device driving the independent motor by delivering a suitable electrical current is able to adapt to changing demands and forces with much more flexibility than a mechanical device. The control device can vary the characteristics of the acceleration phase, of the deceleration phase and of the braking phase of the conveying device with greater consistency, simplicity and ease than a mechanical device, as well. The conveying by means of the independent motor in particular, permits better adjustment of the relative duration of the moving and braking phases of the gripper bar chains with respect to the moving and braking phases of the die cutting platen. Such an arrangement can decrease the duration of the platen braking phase, which permits an increased machine production rate.